Billionaire investor Warren Buffett and the CEO of credit rating agency Moody's are scheduled to face questions Wednesday from a bipartisan panel probing the roots of the financial crisis.

The congressionally chartered Financial Crisis Inquiry Commission (FCIC) subpoenaed Buffett to appear at the New York hearing on the credibility of credit ratings and the investments made based on those ratings.

Rating agencies like Moody's, Standard & Poor's and Fitch Ratings have been criticized for giving unrealistically high ratings to complex investments backed by risky mortgages and other assets. When homeowners started defaulting on their mortgages, the rating agencies downgraded billions of dollars of investments at once. That helped spark the financial crisis.

Despite his company's stake in Moody's, Buffett himself has said he never relies on credit ratings when making investment decisions because he makes his own judgments on companies.

The FCIC issued its first subpoena in April to Moody's, saying the company failed to provide documents it requested. FCIC chairman Phil Angelides said the company started to comply with the request after receiving the subpoena.

At Wednesday's hearing, the panel will probe how the agencies decide on their ratings, how those ratings contributed to the financial crisis and whether the agencies' business model is partly to blame.

The government has filed civil fraud charges against Goldman, alleging it failed to tell investors that one of its clients, hedge fund Paulson, was betting against the securities.

Credit rating agencies came under fire in April from the Senate permanent subcommittee on investigations, which is also probing the causes of the financial crisis. The panel's chairman, Senator Carl Levin, said the Senate's regulatory overhaul should curb the industry's inherent conflicts of interest that allow rating agencies to be paid by the banks whose investments they rate.

Banks generally want higher ratings to make the securities they offer more attractive to investors, and former executives have acknowledged that competition within the industry often led the agencies' analysts to rate high-risk securities as safe.

To tackle the conflict of interest problem, the Senate's version of the financial overhaul would end banks' ability to choose the agencies that rate their investments. An independent board, appointed by regulators, would choose the rating firms.

But critics of that plan point out that the agencies would still be paid by the banks whose products they rate. That means the ratings could be influenced by those banks.

Others have questioned whether regulators – who themselves missed warning signs leading to the crisis – should choose which agencies rate which financial products.

The FCIC will also hear testimony from several former Moody's executives at Wednesday's hearing.